Inserting "Page Break"

The image you’ve posted looks like you’re looking for a horizontal rule, which is a line across the content. The html tag is <hr />. Read more here: http://www.w3schools.com/tags/tag_hr.asp

There’s no button in the standard WordPress menu to add a horizontal rule, so you have to switch to the “text” tab in the editor and add it to the code there to get it to appear.